How NASA brought the monstrous F-1 “moon rocket” engine back to life

The story of young engineers who resurrected an engine nearly twice their age.

There has never been anything like the Saturn V, the launch vehicle that powered the United States past the Soviet Union to a series of manned lunar landings in the late 1960s and early 1970s. The rocket redefined "massive," standing 363 feet (110 meters) in height and producing a ludicrous 7.68 million pounds (34 meganewtons) of thrust from the five monstrous, kerosene-gulping Rocketdyne F-1 rocket engines that made up its first stage.

At the time, the F-1 was the largest and most powerful liquid-fueled engine ever constructed; even today, its design remains unmatched (though see the sidebar, "The Soviets," for more information on engines that have rivaled the F-1). The power generated by five of these engines was best conceptualized by author David Woods in his book How Apollo Flew to the Moon—"[T]he power output of the Saturn first stage was 60 gigawatts. This happens to be very similar to the peak electricity demand of the United Kingdom."

Despite the stunning success of the Saturn V, NASA's direction shifted after Project Apollo's conclusion; the Space Transport System—the Space Shuttle and its associated hardware—was instead designed with wildly different engines. For thirty years, NASA's astronaut corps rode into orbit aboard Space Shuttles powered by RS-25 liquid hydrogen-powered engines and solid-propellant boosters. With the Shuttle's discontinuation, NASA is currently hitching space rides with the Russians.

But there's a chance that in the near future, a giant rocket powered by updated F-1 engines might once again thunder into the sky. And it's due in no small part to a group of young and talented NASA engineers in Huntsville, Alabama, who wanted to learn from the past by taking priceless museum relics apart... and setting them on fire.

Enlarge/ An F-1 engine on display at NASA's Marshall Space Flight Center. Author's wife at right for scale.

Lee Hutchinson

Enter our young rocket scientists

Tom Williams is the kind of boss you want to have. He's smart, of course—that's a prerequisite for his job as the director of the NASA Marshall Space Flight Center's (MSFC) Propulsion Systems Department. But he doesn't mind stepping back and giving his team interesting challenges and then turning them loose to work out the details. Case in point: NASA's Space Launch System (SLS), intended to be an enormous heavy-lift system that will rival the Saturn V in size and capabilities. In thinking about propulsion for the SLS, NASA for the first time in thirty years is considering something other than solid rocket boosters.

The decision to use a pair of solid rocket boosters for the Space Shuttle instead of liquid-fueled engines like the F-1 had been partly technical and partly political. Solid fuels are hugely energy dense and provide an excellent kick to get a spacecraft moving off of the ground; also, selecting solid fuel boosters allowed the government to send some available contracting dollars to companies involved with building intercontinental ballistic missiles, leveraging that expertise and providing those companies with additional work.

The Soviets

The closest thing the Saturn V had to a contemporary was the Soviet N1, which launched four times and exploded each time, almost always because of failures in the complex system that managed the N1's 30 individual first-stage rocket motors. In contrast, the Saturn V has an unblemished string of successful launches, never suffering a problem or failure significant enough to trigger an abort.

Though the F-1 was the largest and most powerful single-chamber liquid-fueled rocket engine ever successfully flown, its power was exceeded by a pair of Soviet designs. The RD-170 engine (used for the only two launches of the Energia rocket) and its RD-171 variant (used on the Zenit rocket) both produce more thrust, but the Soviets were unable to overcome problems with combustion instability in a large rocket's nozzle. Combustion instability is the tendency of the burning propellent to swirl as it is pumped into the nozzle; as we'll see, NASA eventually developed a series of baffles on the F-1's injector plate to damp its instability. The Soviet Union chose to work around the problem by fitting the RD-170 with four separate nozzles instead of one large one, giving the RD-170 and -171 the visual appearance of being four separate engines.

This workaround came to be not because the Soviets were lesser engineers or scientists—the Soviet space program was filled with brilliant, talented people—but instead because for most of its existence the Soviet space program's various rocket design bureaus were caught in a push-pull war of direction and leadership between two chief designers: Sergei Korolev and Valentin Glushko. Korolev favored cryogenically fueled rockets, and Glushko favored those powered by hypergolic propellants. This split in rocket strategy sapped resources, preventing the full force of Soviet engineering talent from focusing on either and ultimately stunting their rocketry program.

For more information on the clashes between Korolev and Glushko, see Asif Siddiqi's Challenge to Apollo (also available in twoparts from various places). It is the definitive work on the history of the fascinating and curiously fragmented Soviet space program.

But solid boosters have several downsides, including an inability to stop combustion. Without pumps to switch off or valves to close, solid boosters work a lot like the "morning glory" sparklers my dad used to buy on the Fourth of July—once lit, they burn until they're done. Solid rocket booster design decisions, specifically in regard to containing combustion, contributed to the destruction of the Space Shuttle Challenger and the death of its crew (though Challenger's destruction was more a failure of NASA management than of technology).

Still, as the Space Shuttle program drew to a close and potentialsuccessors came and went, the inertia of solid boosters and the facilities and people that produced them ensured that they remained a part of the plans.

SLS gave NASA the chance to do a total rethink. As design studies got underway, Williams realized it might be a good idea to re-familiarize the MSFC Propulsion Systems Department with huge kerosene gas generator engines like the F-1 (referred to in shorthand as "LOX/RP-1" or just "LOX/RP" engines, after their oxidizer and fuel mixture of liquid oxygen and RP-1 kerosene). Scale aside, the F-1 is conceptually a relatively simple design, and that simplicity could translate into cost reduction. Reducing cost for space access is a key priority—perhaps even the overriding priority—outside of safety.

There was a problem, though. SLS' design parameters called for a Saturn V-scale vehicle, capable of lifting 150 metric tons into low Earth orbit. No one working at MSFC had any real experience with gigantic LOX/RP-1 engines; nothing in the world-wide inventory of launch vehicles still operates at that scale today. So how do you make yourself an expert in tech no one fully understands?

Nick Case and Erin Betts, two liquid engine systems engineers working for Williams, found a way. Although no launch vehicles that used F-1 engines are still around, actual F-1s do exist. Fifteen examples sit attached to the three Saturn V stacks on display at NASA facilities, including MSFC; dozens more are scattered around the country on display or in storage. Williams' team inspected the available engines and soon found their target: a flight-ready F-1 which had been swapped out from the launch vehicle destined for the to-be-canceled Apollo 19 mission and instead held in storage for decades. It was in excellent condition.

Case and Betts spearheaded the paperwork-intensive effort to requisition the F-1 from storage and get it into their workshop. They were aided by R.H. Coates, a more senior member of Williams' team and lead propulsion engineer for the SLS Advanced Development Office. Williams offered encouragement and assistance from the management side, but the team was otherwise given free rein on how to proceed. After some study, they came to Williams with a request that was pure engineer: "Why don't we just go ahead and take this thing apart and see what makes it work?"

Williams said yes. "It allowed some of our young engineers to get some hands-on experience with the hardware," he told me, "what we would term the 'dirty hands' approach to learning, just like you did when you took apart your bicycle when you were a kid, or your dad's lawnmower or his radio. One of the best ways to learn as an engineer, or in anything, is to take it apart, study it, ask questions."

And then, hopefully, build a better one.

The plans! The plans!

The F-1 teardown started in relatively low-key fashion. As the team dug into the engine, it became obvious that the internal components were in good shape. In fact, though there was some evidence of rainwater damage, the engine overall was in great shape.

The team initially wanted to build an accurate computer model of every component in the engine so that its behavior could be modeled and simulated, but another goal soon began to take shape: maybe, just maybe, they could mount some of the engine components on a test stand and make the F-1 speak again after 40 years.

Why was NASA working with ancient engines instead of building a new F-1 or a full Saturn V? One urban legend holds that key "plans" or "blueprints" were disposed of long ago through carelessness or bureaucratic oversight. Nothing could be further from the truth; every scrap of documentation produced during Project Apollo, including the design documents for the Saturn V and the F-1 engines, remains on file. If re-creating the F-1 engine were simply a matter of cribbing from some 1960s blueprints, NASA would have already done so.

A typical design document for something like the F-1, though, was produced under intense deadline pressure and lacked even the barest forms of computerized design aids. Such a document simply cannot tell the entire story of the hardware. Each F-1 engine was uniquely built by hand, and each has its own undocumented quirks. In addition, the design process used in the 1960s was necessarily iterative: engineers would design a component, fabricate it, test it, and see how it performed. Then they would modify the design, build the new version, and test it again. This would continue until the design was "good enough."

Further, although the principles behind the F-1 are well known, some aspects of its operation simply weren't fully understood at the time. The thrust instability problem is a perfect example. As the F-1 was being built, early examples tended to explode on the test stand. Repeated testing revealed that the problem was caused by the burning plume of propellent rotating as it combusted in the nozzle. These rotations would increase in speed until they were happening thousands of times per second, causing violent oscillations in the thrust that eventually blew the engine apart. The problem could have derailed the Saturn program and jeopardized President Kennedy's Moon landing deadline, but engineers eventually used a set of stubby barriers (baffles) sticking up from the big hole-riddled plate that sprayed fuel and liquid oxygen into the combustion chamber (the "injector plate"). These baffles damped down the oscillation to acceptable levels, but no one knew if the exact layout was optimal.

Enlarge/ Detail on an F-1 engine injector plate at the forward end of the nozzle. Fuel and liquid oxygen are sprayed out of these holes under tremendous pressure, with each ring alternating propellant and oxidizer. Photo is from F-1 engine number F-6045, on public display at the US Space and Rocket Center in Huntsville.

Lee Hutchinson

The baffle arrangement "was just a trial and error thing," explained Senior Propulsion Engineer R.H. Coates. "But we'd like to model that and say, well, what if you took one of those baffles out?" Because the baffles are mounted directly to the injector plate, they take up surface area that would otherwise be occupied by more injector holes spraying more fuel and oxidizer; therefore, they rob the engine of power. "So if you want to up the performance on this thing, we can evaluate that with modern analytical techniques and see what that does to your combustion stability."

But before any "hot-fire" testing could occur, the team had to take the very physically real F-1 engine and somehow model it. It's easy—well, relatively easy—to turn a set of CAD files into a real product. Turning a real product into a set of CAD files, though, requires a bit of ingenuity, especially when that product is a gigantic rocket engine.

To tackle the task, NASA brought in a company called Shape Fidelity, which specializes in a technique called "structured light scanning." If you don't have access to the laser from TRON, structured light scanning is just about the next best way to cram something inside of a computer.

Lee Hutchinson / Lee is the Senior Reviews Editor at Ars and is responsible for the product news and reviews section. He also knows stuff about enterprise storage, security, and manned space flight. Lee is based in Houston, TX.